Excessive S-adenosyl-L-methionine-dependent methylation increases levels of methanol, formaldehyde and formic acid in rat brain striatal homogenates: possible role in S-adenosyl-L-methionine-induced Parkinson's disease-like disorders.
OBJECTIVE
Excessive methylation may be a precipitating factor for Parkinson's disease (PD) since S-adenosylmethionine (SAM), the endogenous methyl donor, induces PD-like changes when injected into the rat brain. The hydrolysis of the methyl ester bond of the methylated proteins produces methanol. Since methanol is oxidized into formaldehyde, and formaldehyde into formic acid in the body, we investigated the effects of SAM on the production of methanol, formaldehyde and formic acid in rat brain striatal homogenates and the toxicity of these products in PC12 cells.


METHODS
Radio-enzymatic and colorimetric assays, cell viability, Western blot.


RESULTS
SAM increased the formation of methanol, formaldehyde and formic acid in a concentration and time-dependent manner. Concentrations of [3H-methyl]-SAM at 0.17, 0.33, 0.67 and 1.34 nM produced 3.8, 8.0, 18.3 and 34.4 fmol/mg protein/h of [3H] methanol in rat striatal homogenates, respectively. SAM also significantly generated formaldehyde and formic acid in striatal homogenates. Formaldehyde was the most toxic metabolite to differentiated PC12 pheochromocytoma cells in cell culture studies, indicating that formaldehyde formed endogenously may contribute to neuronal damage in excessive methylation conditions. Subtoxic concentration of formaldehyde decreased the expression of tyrosine hydroxylase, the limiting factor in dopamine synthesis. Formaldehyde was more toxic to catecholaminergic PC12 cells than C6 glioma cells, indicating that neurons are more vulnerable to formaldehyde than glia cells.


CONCLUSIONS
We suggest that excessive carboxylmethylation of proteins might be involved in the SAM-induced PD-like changes and in the aging process via the toxic effects of formaldehyde.
Lee
Eun-Sook
ES
Department of Neurology, Meharry Medical College, Nashville, TN 37208, United States. elee@mmc.edu
Chen
Hongtao
H
Hardman
Chadwick
C
Simm
Anthony
A
Charlton
Clivel
C
D000818:Animals; D015153:Blotting, Western; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D003342:Corpus Striatum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D005557:Formaldehyde / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D005561:Formates / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D008297:Male; D000432:Methanol / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology; D008745:Methylation; D020734:Parkinsonian Disorders / Q000201:enzymology / Q000209:etiology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012436:S-Adenosylmethionine / Q000378:metabolism / Q000494:pharmacology* / Q000502:physiology; D014446:Tyrosine 3-Monooxygenase / Q000096:biosynthesis
D005561:Formates; C030544:formic acid; D005557:Formaldehyde; D012436:S-Adenosylmethionine; D014446:Tyrosine 3-Monooxygenase; D000432:Methanol